This invention relates to the fabrication of detailed dental appliances and more particularly to a method of designing and making orthodontic brackets and the like.
Heretofore, orthodontic brackets have customarily been made by die stamping, machining or casting. As the orthodontic art has progressed, the desire to make more intricate and smaller parts has been limited by the available materials and methods of fabrication.
In the past, gold was the material of choice for orthodontic brackets, but now stainless steel is usually used. The 300 series stainless steels are the most common materials selected for orthodontic attachments. Sometimes 17/7 and 17/4 stainless steels are used because these steels can be heat treated.
The tooling for stamped parts is expensive, but once it is built, the parts can be made relatively accurately and inexpensively. Stamping and forming requires the use of softer metals but this limitation can be overcome by using heat treatable metal such as 17/7 stainless steel in the annealed state. The part is then heat treated. Holes in stamped parts must be no smaller than the material thickness. In general, the more complex the parts, the more impractical die stamping and forming becomes.
Job shops that can make shapes in flat stock are readily available, but the stamping and metal folding needed to make complicated orthodontic attachments requires special manufacturing skills. At the present time, use of die stamping and forming is limited to flat parts such as lock pins and mesh foil bonding pads and to the manufacture of simple formed brackets for the Begg light wire technique and for some buccal tubes.
Metal machining is commonly used in the manufacture of orthodontic parts. Semi-soft, machineable grades of 300 series stainless are the preferred materials. Parts made from machineable grade stainless steel cannot be heat treated. Since these parts must remain semi-soft, strength becomes a problem in the manufacture of small cross sections.
Some orthodontic attachments such as a simple lingual button (FIG. 1) can be made on a screw machine. Screw machine job shops are readily available. Set up charges for a screw machine operation are inexpensive, but the cost per part is expensive. As the shapes of attachments become more innovative and complex and less available in the common job shop, the complicated tooling becomes very expensive.
The introduction of the lost wax investment casting process in the manufacture of orthodontic attachments has allowed parts to be designed that could not be made by die stamping and forming or by machining technologies. Wildman's Edgelok bracket (U.S. Pat. No. 3,780,437) is such an example. Orthodontic brackets made by the investment casting method begin as plastic patterns formed by injection molding. Very often this is high density polystyrene. The injection molds in which these patterns are formed are expensive to make but produce very accurate parts. Very intricate shapes can be produced. The investment process is time consuming and the cost per part is high. All parts are fully annealed as cast and are soft. If strength is needed, a heat treatable stainless such as 17/4 can be used. Investment casting to the tolerances required in the manufacture of orthodontic parts is very exacting and is not readily available from commercial job shops.
Sinter bonded powered metal is used in an injection mold and is a viable alternative to casting. A cost advantage in sinter bonding comes from the elimination of the investment step. Nonetheless, the molds must withstand the wear problem of molding the powered metal and are very expensive. Casting limits the design of brackets to shapes and cross sectional dimensions that can be reliably cast without voids and with sufficient strength for orthodontic applications.
Conventionally, bracket bodies are fabricated separately and then mounted as a discrete step on a mesh-foil bonding pad. This is typically done by spot-welding or by brazing with various solders, including 80-20 gold-copper and 82-18 gold-silver eutectics applied in wire or paste form.
The tendency in the art is to move toward automatic and releasable brackets and to applications, such as lingual orthodontics, that require a wider variety of part configurations. The increased complexity challenges the limits of casting techniques and materials. And, together with the need for many different parts, it makes the cost of manufacture prohibitive. It would be desirable for orthodontists to be able to have parts made on a custom or semi-custom basis. Clearly, however, the cost and complexity of making state-of-the-art orthodontic parts by current methods precludes having parts made locally.
Chemical etching, which is sometimes called chemical milling, is a metal forming technology that is known and used in other arts. Chemical milling is used extensively in the production of flat stainless steel parts and is readily available from commercial job shops. It is also used in the electronics industry for making printed circuit boards. In this process, a photographic tool or mask is made from a line drawing and an acid resistant layer is printed on a sheet of metal. When the sheet is dipped in an acid bath, the imprinted pattern is resolved in the acid, leaving untouched the metal that is protected by the resistant film. This process is relatively cheap. Very intricate complicated shapes can easily be produced. Hard materials can be shaped by chemical etching. By staggering the pattern of the printed resistant film, some areas can be etched on one side but not on the other. Chemical etching is limited to relatively thin flat stock.
In its conventional form, chemical etching is unsuited for making orthodontic attachments other than such things as lock pins made from flat stock. Accordingly, a need remains for a better process for making orthodontic brackets.